Beyond that, this isn't just about how she feels about it. Everyone she knows, relatives, friends, is going to want to see the ring and hear the story. To all of them, a diamond is the gold standard and a cheap substitute will be an awkward counterfeit. Even if you convince her, you are signing her up to have the ultimate celebration of her, the once in a lifetime birthday party, turn into a lot of awkwardness and sympathy. Bad deal.

Ignore the cost. It's impolite to ask and moissanite is expensive enough. If what you do looks legitimate enough, no one need know. The critical key is that you sell this as better, representative of more thought, more effort, more investment on your part.

Focus on that you're doing something different. Something special. Design a custom ring that speaks to you about who she is. Focus on the ways in which the stone is superior to a diamond. It is unusually brilliant which reminds you of her. It is tougher, harder than what everyone else takes for granted. You mounted it with blue stones for her eyes.

Make it personal and in every possible way, about her and as much as possible tell a story that will make her friends wish someone that thoughtful cared about them enough to do something truly special rather than checking the expected box.

First and foremost, it's a proof of social status between her and her peers, so you need to know who her friends are. But there are so many factors to weigh in. Someone I know got a sapphire ring to propose to his girlfriend, and she commented about how she was glad that he remembered that she hated diamonds. A couple I know have silver wedding bands which were made by an artist that they both know. The rings are unique and special—and this is important—in a way that they can brag about. They don't brag about it, but they could, if they were that kind of people.

However, if you are unable to express your concerns about diamonds without worrying that she'll think you love her less, then you are NOT ready to propose. Consider premarital counseling—which is something you do to get ready for marriage, not something you do because you have problems.

Charles and Colvard (the originators of lab grown moissanite) recently released a new generation known as Forever One and these cuts are currently the best on the market. They're brighter and have more sparkle than diamonds and are completely colourless. Essentially, they look better than any diamond money can buy and have the same properties in terms of hardness etc.

Despite all the advantages of moissanite (quality, price, ethics, environment etc), I knew a little part of her still wanted a real diamond on her finger (no matter how much she tried to pretend otherwise!).

My solution looked like this:

1. A 1 carat 6.5mm Forever One moissanite round centre stone (in a halo setting)

2. ~20 tiny, 0.3 carat real side diamonds (F colour, SI clarity - Note: these two metrics are much more important than the carat weight!)

3. 18K gold for the ring itself (bonus: the gold was recycled - feeble but worthwhile attempt to offset the environmental damage of the diamonds)

All told, it came in at under $1,500 (which, sadly, is ridiculously cheap these days for an engagement ring).

She loves it (and got her diamonds), we have more money to spend on other things and I'm happy I didn't lose too much cash and karma to the diamond cartels.

FYI, I did all of this online through https://www.moissaniteco.com. Highly recommend them!

Other tip, her and I went ring shopping together, which really helped alleviate the pressure of trying to pick out something she'd enjoy wearing for many decades. She was still pleasantly surprised by the prosposal a couple months later, although she obviously knew it was in the cards at some point!

We had already had conversations about getting married, that sort of thing, so when it came to the ring it was easy to bring it up. I mentioned I was a little reluctant to go with a diamond; I said I was -willing-, but due to the intentional supply limit and the ensuing price hike, it would necessarily be small, as I was not willing to go into debt for it (which she was already in agreement we should never do; beware if your significant other has no issues with going into debt). Had she considered alternatives, such as moissanite?

I showed her some images and videos of moissanite, compared price, showed it was nearly as hard -and- glittered more (and that I could get her a 2+ carat stone for what ~.5 carat diamond of comparable color and clarity would cost, and that at < 1/10th what a 2 carat diamond of comparable color and clarity would cost), and she was convinced enough to go scope it out at a jeweler's that carried it.

I also mentioned the possibility of gemstones; there were plenty that were gorgeous and would be more distinctive than diamond, and set her apart from other women (who, after all, could only brag about the size of their diamond, rather than how it had special meaning to them). Etc.

At the jeweler's as she explored different stones, she ended up picking out an aquamarine that she really liked (and then found out that was my birthstone, locking in the decision even more).

66% of marriages survive 10 years. The ring would cost 50c per day.

Buy the ring, and spend less of coffee.